When working with collections of linked entities, such as documents that are accessible on the portion of the Internet known as the World Wide Web (hereinafter “the Web”), users generally desire to identify relevant documents, yet often have to manually sift through many documents in order to distinguish between relevant and irrelevant documents. In order to help users find relevant documents on the Web more efficiently, search engines, for example, have been designed to rank documents found in response to a query according to an estimation of relevance. In one such estimation technique, a document's relevance is deemed to be proportionate to the number of other documents that are linked to the document, either directly via a hypertext link, or indirectly via a chain of hypertext links.
In printed publications, such as scientific journals, time is considered one of the most important factors in distinguishing between subjects that are obsolete and those that are emerging. Relevance over time is a measure used by libraries to rank journals as part of their decision whether to subscribe or unsubscribe. Authors of scientific papers often decide where to publish their papers based on the current popularity of a journal, the importance of citations made to the journal, and how recently citations were made to the journal.
It has been shown that citations of journal articles behave in a consistent manner. In general, the more time passes the less citations a paper receives. In general, a journal will be considered more prominent the higher its citation half-life, i.e., how old are most of the papers currently cited in the literature that were previously published in the journal. A journal's impact-factor, or the frequency with which the average article in a given journal has been cited in a particular year, is also used by libraries to determine a journal's value. Since a journal's value can change over time, this evaluation is carried out periodically.
In contrast, when plotting similar measures for citations on the Web, the reverse behavior is exhibited: the more time passes the more citations a Web document receives. Furthermore, unlike printed publications, Web documents are often modified and updated with respect to current events. For example, when a country's government changes, the address of the official government Web site remains the same while its content is changed according to the wishes of the new government. This is in contrast to printed publications that become obsolete, or stale, over time and are then replaced by new, fresh documents.
Thus, in addition to the application of the aforementioned time-based relevance estimation techniques for printed publications not being known for Web-based documents, the manner of their application is not self-evident due to the disparate nature of Web-based documents and printed publications.